Cisco Router Booting Process

1) As we know that ROM stand for Read Only Memory. It stores four components POST, Bootstrap program, ROMMON mode and Mini IOS.

2) POST stand for Power On Self Test, it is a low level diagnostic program utility that performs various tests on hardware components. It verifies that all necessary components are present and operational.

3) Bootstrap program is the second utility in booting sequence. It controls the search and load process of IOS. Bootstrap program is responsible for finding IOS on all possible locations and loading it in RAM.

4) ROMMON is a portable IOS program that allows us to perform various diagnostic tests. This program is also used for password recovery procedure. It has its own mode known as ROMMON mode. If bootstrap successes in finding and loading operation of IOS, then boot sequence will not enter in this mode. Boot sequence will enter in this mode automatically, if it fails to load IOS in RAM from all possible locations. You can manually enter in this mode for diagnostic purpose. Run reload command from privileged mode to reboot the router. Press + C key combination ( CTRL Key with C Key ) in first 60 seconds of boot sequence.

5) Mini-IOS is a fallback utility that contains a stripped down version of IOS. This is used in critical situations where IOS image in flash is not found. Mini-IOS contains only IP code that allows you to load IOS from other resources such as TFTP Server. Cisco IOS mode used by this stripped down IOS utility known as RXBOOT mode.

6) Flash is a nonvolatile memory. Data store in flash is not lost when you turn off the router.

7) NVRAM is another permanent memory in router. Data stored in NVRAM is also remain safe. Router use NVRAM to store configuration files.

8) RAM is a temporary memory. Information stored in RAM does not remain in power off stage. Everything in RAM is erased, when you turn off the router. RAM is the fastest memory among these memories.

A. RAM is responsible for decompressed IOS during the boot process.

B. During the boot process running configuration is also copied in RAM from NVRAM.

C. RAM is responsible for store ARP, CDP, Routing and Neighbor table.

D. Interface input and output buffers are also stored in RAM.

Configuration register value is a special register that is used to control the booting process. You can check current configuration register value with "show version" command from exec mode .

A. Setting of 0x2100 will always boot router in ROMMON mode.

B. Setting of 0x2101 will always boot router in RXBOOT mode.

C. Setting of 0x2102 to 0x210F will always load first valid IOS from flash.

D. The setting of 0x2142 causes the router to ignore the startup-config file in NVRAM and proceed without a configuration. Thus the setting of 0x2142 is used for password recovery process of router.

[1]

Router is powered on.

[2]

Power on self test (POST) is performed to check hardware components including memory and interfaces.

[3]

Bootstrap program is loaded and executed.

[4]

Bootstrap reads configuration register value to determine how the router will boot up.

[5]

Depending on the value of configuration register, bootstrap program finds and loads the IOS image.

[6]

If bootstrap fails to load IOS from all possible locations it will drop boot sequence in ROMMON mode for troubleshooting.

[7]

If IOS is loaded, the IOS will try to find and load the configuration.

[8]

If configuration is not presented, system configuration dialog would be launched.

[9]

If configuration is loaded, you would be presented in CLI interface.